The present invention relates to forming images of the heart from multiple sets of ultrasound data.
Ultrasound is widely used for cardiac imaging. The quality of cardiac ultrasound images is determined mainly by the size and acoustic transparency of the acoustic windows through which the heart can be imaged. The rib cage offers multiple spatially distinct acoustic windows, but each window has a small area. Therefore transducers used in Cardiology applications typically have small apertures, such as 19.2 millimetersxc3x9714 millimeters. As a result, the detail resolution and field of view are limited and the ability to detect directional targets, such as endocardial borders, is impaired. The limited access also means poor signal-to-noise ratio (SNR). The poor SNR may particularly impact those modes that inherently have low SNR, such as the harmonic imaging. During the course of an ultrasound cardiac exam, a transducer is scanned across the rib cage to image from various spatially separated acoustic windows. The images from different acoustic windows are viewed separately. Therefore, information content determined by the field of view, detail and contrast resolution and SNR is limited by the individual acoustic window size and quality.
Another important factor that determines the quality of cardiac ultrasound imaging is the frame rate. Valves and other quickly moving objects within a heart may be poorly imaged due to insufficiently low frame rates. Therefore techniques that require multiple firings per beam such as certain types of compounding and sequential focus are typically not used for cardiac ultrasound.
Three-dimensional cardiac imaging has also been provided. Leotta et al. provide for three-dimensional cardiac imaging using multiple transthoraxic acoustic windows as described in xe2x80x9cQuantitative three-dimensional echocardiography by rapid imaging from multiple transthoraxic windows: in vitro validation and initial in vivo studies,xe2x80x9d Journal of the American Society of Echocardiography, Vol. 10, No. 8, pp. 830-839 (1997). A two-dimensional image of the heart is acquired through each of multiple transthoracic windows. Regions or 3-D surfaces within each image are detected. Using magnetic positions sensors on a transducer, the relative position of each of the borders to other detected borders is used to generate a surface. The surface is then rendered in two dimensions for display to the user.
Other techniques for combining ultrasound images are known. U.S. Pat. No. 6,014,473 discloses compounding detected images associated with different transducer array positions to form an extended field of view. Images associated with different array or aperture positions may also be combined prior to detection. Spatial compounding may also be provided combining images associated with steering scan lines at different angles to a stationary array. The technique of combining images from different acoustic windows before the amplitude detection is known as the synthetic aperture, while combining such images post-detection is known as spatial compounding or compound aperture. The synthetic aperture reduces the size of speckle, which is the image of irresolvable targets, and increases the detail resolution of resolvable targets. The compound aperture, on the other hand, reduces the variance of speckle and improves detectability of directional targets U.S. Pat. Nos. 6,132,374 and 6,048,316 discloses sequential focus or compounding images associated with different focal depths. U.S. Pat. No. 5,961,460 discloses compounding images associated with different center frequencies. There is a need for ways to apply these techniques to imaging a dynamic object like heart, which is accessible from multiple spatially distinct acoustic windows.
The present invention describes forming images of the heart by acquiring and processing multiple sets of ultrasound data where each set is responsive to a different set of imaging parameters. The imaging parameter sets differ in at least one parameter, such as array position, temporal frequency response or transmit focal depth, so that the images formed using these data sets have, either laterally or axially, different spatial spectra. The multiple images thus formed are combined to improve contrast resolution and field of view.
In one aspect, a set of images is formed responsive to a first imaging parameter set for a first cardiac cycle. Then another set of images is formed responsive to a second imaging parameter set for a second cardiac cycle. Alternatively, the two sets of images are formed for a sub-cycle of interest, rather than for a full cardiac cycle, using first and second cardiac cycles. Then, the two sets of images are temporally aligned so that they correspond to the same set of phases of the cardiac cycle. The temporally aligned images may also be spatially aligned and combined.
One of the imaging parameters of interest that is varied between the two data acquisition events is the array position. Combining images of anatomical structures common to spatially distinct array positions can improve the contrast resolution, and/or assembling (e.g. volume rendering) the images of anatomical structures uncommon to the different array positions can extend the field (e.g. volume or area) of view. In one of the embodiments, the multiple data sets are acquired sequentially by acquiring a first set from an acoustic window, manually translating the transducer across an acoustically opaque barrier such as a rib, acquiring a second set and so on. The translation is done such that the imaging plane is substantially preserved. In another embodiment, a transducer with multiple spatially distinct coplanar arrays is used to acquire the multiple data sets. To get the benefit of spatial compounding the multiple arrays are arranged such that their imaging planes are the same, or, in the case where each array is a 2-D array, at least one of the imaging planes is common to both arrays. The separation and relative angle of these arrays may be adjustable to accommodate patient-to-patient and view-to-view variations of the acoustic window position and angles.
Forming an image by combining multiple images where each image is responsive to a different array position requires spatial alignment of the images prior to combining. Since the heart is dynamic, the sets of images are also temporally aligned prior to combining. Therefore, methods of spatial and temporal alignment of the multiple images are also described.
Another imaging parameter of interest that is varied between the two data acquisition events is the imaging center frequency. Combining images at different imaging center frequencies (e.g. frequency compounding) improves contrast resolution by reducing speckle variance. Yet another imaging parameter of interest is transmit focal depth. Combining images responsive to different transmit focal depths (e.g. sequential focus), extends the depth of field.
Improved images of objects associated with cyclical motion are provided without sacrificing temporal resolution by distributing the processing required for techniques such as frequency compounding, spatial compounding or sequential focus over multiple cycles of the motion. These methods and systems are applicable to 2-D or 3-D imaging of any dynamic object with cyclic motion, and/or objects that offer multiple spatially separated acoustic windows.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.